1. Field of the Invention
The present invention relates to a thread protector for use with a pipe, and more particularly, to a thread protector for protecting a male thread or a female thread formed to an end of the pipe.
2. Description of the Related Art
Ordinarily, oil well pipes used in oil fields and the like must have a length of several thousand meters. Such a long oil well pipe is formed by coupling pipes each having a male thread or a female thread formed to the ends thereof. The coupling portion of the thread is required to keep gas tightness even if external force applied to the pipe, and pressure, heat, and the like from the inside of the pipe act on the coupling portion and is further required not to be deformed and broken thereby.
When the male thread of a pipe is tightened to the female thread of another pipe in the coupling portion thereof, if the threads are composed of trapezoidal threads, they are tightened in the following two states depending on a facility in which the pipes are used.
As shown in FIG. 7, in a first state, a male thread 3b and a female thread 3a of two pipes are tightened to each other with the guide flank surfaces (stub flank surfaces) 16b and 16a thereof in contact with each other. In this state, on the female thread 3a side, the guide flank surface 16a and the top surface (crest surface) 12a thereof are subjected to friction under the bearing stress of the male thread 3b. Further, on the male thread 3b side, the guide flank surface 16b and the bottom surface (root surface) 14b thereof are subjected to friction under the bearing stress of the female thread 3a. 
As shown in FIG. 8, in a second state, a male thread 3b and a female thread 3a of two pipes are tightened to each other with the load flank surfaces 18b and 18a thereof in contact with each other. In this state, on the female thread 3a side, the load flank surface 18a and the top surface 12a of the female thread 3a are subjected to friction under the bearing stress of the male thread 3b. Further, on the male thread 3b side, the load flank surface 18b and the bottom surface 14b thereof are subjected to friction under the bearing stress of the female thread 3a. 
Further, when the male thread 3b is centered to the female thread 3a before they are tightened with each other, the corner portions formed by the guide flank surfaces 16b and 16a and the top surfaces 12b and 12a of the male and female threads 3b and 3a may collide with each other as shown in FIG. 9.
Conventionally, the friction of the flank surfaces, and the like and the collision of the corner portions in the above states in which the threads are tightened are coped with using grease. That is, grease is applied to the thread portions of pipes to suppress the metal surfaces of a male thread and a female thread from coming in contact with each other and further reduces friction when the threads are tightened to each other by the lubrication effect of the grease, thereby the damage and galling (seizing) of thread surfaces are prevented.
However, since grease often contains heavy metals and has a bad decomposing property in the oceans, there have been recently proposed methods of eliminating the use of grease by providing a thread surface with a lubricating property by subjecting a thread to a surface treatment from a view point of preventing environmental pollution and further because of a reason that when a protector is removed in deserts from a pipe having a thread to which grease is applied, sands adhere to the grease and make it difficult to normally tighten the thread.
As a method of eliminating the use of grease, Japanese Patent Application Laid-Open (JP-A) No. 9-72467 discloses a thread joint excellent in a galling resistant property in a non-lubricating state in which neither grease nor a liquid lubricant is used by providing an irregular portion on a thread surface, forming a resin film layer, and flattening the surface of the irregular portion with the resin film layer. Further, JP-A No. 2001-65752 discloses a thread joint for an oil well pipe in which a lubrication film composed of an inorganic polymer compound is formed on the contact portion of the thread joint.
However, these methods of eliminating the use of grease are disadvantageous in that when threads are tightened, since the surfaces of the threads come into direct contact with each other, surface treatment layers formed on the guide flank surfaces, the load flank surfaces, the corner portions, and the like are particularly liable to be subjected to wear and damage.
Further, a pipe having a male thread or a female thread formed to an end thereof has a protector attached to the end to protect the thread or to prevent foreign materials such as sands and the like from entering the pipe. The protector has a thread to be threaded with a male thread or a female thread formed to an end of the pipe, and the protector is rotated on the male thread or in the female thread and attached to the pipe to protect it.
In view of the problem that the surface treatment applied to the specific portions described above is particularly subjected to wear and the like, the protector for protecting the thread is required not to damage the surface treatment of the specific portions. Further, it is preferable to design the protector such that it is not in contact with the specific portions.
JP-A No. 2001-199469 discloses a pipe end protector in which the groove width and the height of a thread portion are prescribed within predetermined ranges as a protector whose structure is prescribed.
However, an object of the invention of JP-A No. 2001-199469 is to optimize the gap between a thread portion formed to a pipe end and a thread portion of the pipe end protector to thereby effectively prevent an untirust agent filling the gap from flowing out without deteriorating workability in tightening and loosing. As described above, the invention disclosed in JP-A No. 2001-199469 uses grease (untirust agent) as a precondition, and the reason for restricting the thread shape of the protector does not intend to protect the surface treatment of the thread portion of the pipe and thus does not satisfy the above requirement.